1. Field of the Invention
The present invention relates to a heteropolyacid type catalyst containing, as catalyst constituent elements, phosphorus, molybdenum, copper and arsenic and suitable for use in the production of methacrylic acid, at a high yield, from methacrolein and molecular oxygen at an elevated temperature under a vapor phase.
2. Description of the Prior Art
It is known in the art that methacrylic acid is produced by the vapor phase reaction of methacrolein and molecular oxygen in the presence of heteropolyacid type catalysts containing as catalyst constituent elements, phosphorus, molybdenum, copper and arsenic, at an elevated temperature. The known heteropolyacid type catalysts are generally prepared by first mixing suitable compounds containing the constituent elements of the catalysts, for example, oxides and phosphates and, optionally, suitable supports, in the presence of, for example, water; drying, generally evaporating to dryness, the resultant mixture in the form of a solution or slurry; and optionally calcining the dried mixture, with or without molding the dried mixture, as disclosed in, for example, U.S. Pat. Nos. 3,998,876, 4,075,123, 4,075,124 and 4,110,369.
Heteropolyacids or their salts containing phosphorus and molybdenum are believed to form a so-called Keggin structure as a skelton structure, which structure is surrounded by protons, or cations, and water of crystallization. Zero to 30 molecules of water of crystallization per molecule of the heteropolyacids is included in the heteropolyacids.